runas will always prompt for a password, but if the account has no password (like Guest) you can just leave it blank and hit enter. The program should run anyway.

If the account you're trying to run as has a password, and you don't know it, you're out of luck. But if you already have administrator access, you can either change that password or create a new user with your desired privilege level to run the command.

Unfortunately there doesn't seem to be a way to automate the password entry, and this is intentional; see this blog entry.